It could mean that you are here to work (depends on context). It definitely means that you are not there to muck around and goof off (or not take seriously why you are there, nor do the expected task in a haphazard, unattentive manner).
Why is the negator "bukan" used for this sentence rather than "tidak" used? I'm trying to understand proper grammar patterns. Is it because the thing being negated is a prepositional phrase (or starting with "untuk" at least)?